1. Field of the invention
The present invention relates to a superconductor of bismuth-containing compound oxide, more particularly, a superconductor of compound oxide of Bi-Sr-Ca-Cu system improved in the critical current density (Jc). The present invention relates also a process for preparing a thin film of such a superconductor.
2. Description of the related art
The critical temperature "Tc" of superconductivity could not exceed 23.2K of Nb.sub.3 Ge which was the the highest Tc for the past ten years. The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 (Z. Phys. B64, 1986 p 189). The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which show such higher Tc as 30K which are extremely higher than known superconducting materials. It was also reported that C. W. Chu et al. discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90K in February 1987 (Physical Review letters, Vol. 58, No. 9, p 908).
It had been known that certain compound oxides containing bismuth exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10K and hence usage of liquidized helium (boiling point of 4.2K) as cryogen was indispensable to realize superconductivity.
Maeda et al reported the other type of new bismuth-containing superconducting material of Bi-Sr-Ca-Cu-O system (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210).
Bismuth type compound oxides are chemically much more stable than the abovementioned YBCO type compound oxide and have such a very important merit that high Tc superconductors of higher than 100K can be realized without using rare earth elements as a constituent element so that the production cost can be reduced.
The above-mentioned new types of superconducting materials were obtained firstly in a bulk form of a sintered block which was obtained by a solid reaction or sintering technique from a powder mixture of oxides and/or carbonates of constituent metal elements which have predetermined atomic ratios.
At the present, it is thought that thin films of these material are much more useful in actual uses. Such thin films can be deposited on a substrate by physical vapour deposition (PVD) technique such as RF sputtering, vacuum deposition, ion-plating or MBE or chemical vapor deposition (CVD) technique such as thermal CVD, plasma CVD, photo-CVD or MOCVD. Usually, the thin films are prepared by RF sputtering technique by using a sintered mass as a target.
It is also a general practice to heat-treat the sintered blocks or thin films in an oxygen-containing atmosphere to adjust the oxygen deficiency in crystals of which sintered blocks or thin films are composed.
The sintered blocks or thin films prepared by the conventional process, however, possessed too low critical current density (Jc) to be used as a material for thin film devices. In fact, the critical current density of the thin film of Bi-Sr-Ca-Cu system obtained heretofore was about 10,000 A/cm.sup.2 at most although its critical temperature (Tc) was as high as more than 100K.
The present inventor found in the course of experiments that the superconducting property, particularly the critical current density (Jc) can be improved remarkably if the amount of bismuth is increased excessively with respect to the stoichiometrical amount and the present invention was completed.
Therefore, an object of the present invention is to overcome the problem of the conventional process and to provide a superconductor of bismuth-containing compound oxide, more particularly, Bi-Sr-Ca-Cu system compound oxide improved in the critical current density (Jc).